Many approaches have been taken to measuring while drilling or logging while drilling. Most approaches have utilized pulsing or vibrating the drilling mud column within the drill pipe for signal transmission.
Any approach utilizing pulses impressed on the mud column is subject to the dampening or attenuating characteristics of the drilling mud such as viscosity, compressibility, density, temperature, entrained air, entrained solids, all of which may serve to some extent to dampen a pulsed or cyclic pressure change within the mud.
One known approach to transmitting signals from the well bore bottom to the earth's surface electrically is disclosed in U.S. Pat. No. 4,057,781. This U.S. Pat. No. 4,057,781, which discloses a system herein referred to as the "cascade" communication system, is herein specifically incorporated by reference for its background information.
The cascade system also involves the method of logging while drilling a well bore.
The attenuating qualities of drilling mud in drill pipe are such that it is postulated that no mud signal will be recoverable at the earth's surface at depth in excess of probably 12-16,000 feet although it has been reported that usable signals have been recovered from depths as great as 12,000 feet.
Also a problem with subsurface telemetry has been that, in large part, batteries are required to operate the detection and transmission circuitry at the bottom of the well bore and also for relaying intermittant signals from places along the drilling string such as in the cascade system above referenced. Batteries are prone to become much less efficient and finally inoperative at temperatures in excess of about 300.degree. F. and, at present, no batteries are commercially available which are operable for any appreciable length of time in some hot, deep wells.
While the cascade system appears somewhat similar to the present invention, such similarity will be seen to be superficial after further examination of the present apparatus and method.
The present invention utilizes the conductive properties of large volumes of the earth formation surrounding the well bore in a manner somewhat similar to wireline open hole electrical logging which may be referred to as "focussed current" logging, or, from another perspective, as "single electrode" logging.